Ligands are broadly classified into two classes classical and non-classical ligands, depending on their donor annd acceptor ability. Classical ligands form classical complexes while non-classical ligands form non-classical complex. Bonding mechanism in non-classical is called synergic bonding.
Q. Which is not `pi`-acceptor ligand?

Options

A.
B. `sigma-C_(5)H_(5)^(-)`
C. `PH_(3)`
D. B_(3)N_(3)H_(6)`

Explanation

To determine which ligand is not a pi-acceptor ligand, we need to analyze the characteristics of each ligand based on the criteria for pi-acceptor ligands. Here’s a step-by-step solution:

### Step 1: Understand the Definition of Pi-Acceptor Ligands
Pi-acceptor ligands are those that can accept electron density from a metal center through pi* (pi star) anti-bonding orbitals or have vacant d-orbitals.

**Hint:** Remember that a ligand must have either a d-orbital or a pi* orbital to be a pi-acceptor.

### Step 2: Analyze Each Ligand
1. **CH (Methyl Group)**:
- Carbon does not have d-orbitals.
- There are no pi bonds, hence no pi* orbitals.
- Conclusion: Cannot act as a pi-acceptor ligand.

2. **C5H5^- (Cyclopentadienyl Anion)**:
- Carbon does not have d-orbitals, but it has pi bonds.
- Since pi bonds are present, pi* orbitals exist.
- Conclusion: Can act as a pi-acceptor ligand.

3. **Ph3 (Triphenylphosphine)**:
- Phosphorus has vacant d-orbitals.
- It can accept electron density.
- Conclusion: Can act as a pi-acceptor ligand.

4. **B3N3H6 (Borazine)**:
- Boron and Nitrogen do not have p-orbitals, but they can have pi bonds.
- The structure has alternating double bonds, allowing for pi* orbitals.
- Conclusion: Can act as a pi-acceptor ligand.

### Step 3: Identify the Answer
From the analysis:
- **CH** is the only ligand that does not meet the criteria to be a pi-acceptor ligand because it lacks both d-orbitals and pi* orbitals.

### Final Answer:
The ligand that is not a pi-acceptor ligand is **CH**.

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